Naphthalenedicarboxylic acids such as 2,6-, 2,7-, 1,5- and 1,4-naphthalenedicarboxylic acid as well as their corresponding dialkylesters can be used to prepare a variety of polyamide and polyester materials. For example, dimethyl-2,6-naphthalenedicarboxylate and 2,6-naphthalenedicarboxylic acid can be reacted with ethylene glycol to prepare poly(ethylene-2,6-naphthalate) (PEN). Fibers and film manufactured from PEN have improved strength and superior thermal properties relative to other polyester materials. Films made from PEN demonstrate, for example, superior resistance to gas diffusion and particularly to the diffusion of carbon dioxide, oxygen and water vapor. Because of its exceptional properties, PEN is especially suitable for applications such as food and beverage containers, particularly for so-called "hot-fill" food and beverage containers, tire cord, magnetic recording tape and electronic components.
Although the dialkyl-2,6-naphthalenedicarboxylates--particularly dimethyl-2,6-naphthalenedicarboxylate--are suitable monomers for preparing PEN and other polymeric materials, in some commercial-scale operations it is preferable to employ 2,6-naphthalenedicarboxylic acid rather than a dialkyl ester. For example, a polyester manufacturer may have equipment and associated processes available for manufacturing polyesters only from an aromatic dicarboxylic acid. In these circumstances, the diester materials would not be suitable and the use of 2,6-naphthalenedicarboxylic acid would be required. Additionally, it is advantageous to use 2,6-naphthalenedicarboxylic acid in the manufacture of polyesters because the condensation of a diacid with a glycol to form a polyester does not form an alcohol by-product as does the condensation of a diester with a glycol. Polyester manufacturers who use diacids such as 2,6-naphthalenedicarboxylic acid do not, therefore, have to provide for the use or sale of the alcohol by-product.
Methods for preparing naphthalenedicarboxylic acids include the bromine promoted, metal catalyzed, liquid phase oxidation of dialkylnaphthalenes. Such processes are disclosed in U.S. Pat. Nos. 3,870,754; 4,950,786 and 4,933,491. The bromine-promoted, metal-catalyzed, liquid phase oxidation of 2,6-dialkylnaphthalenes, particularly 2,6-dimethylnaphthalene, produces a crude product containing a variety of impurities such as brominated 2,6-naphthalenedicarboxylic acids, 2-formyl-6-naphthoic acid, 2-naphthoic acid and trimellitic acid. These impurities, particularly 2-formyl-6-naphthoic acid, are difficult to remove from crude 2,6-naphthalenedicarboxylic acid. The 2,6-naphthalenedicarboxylic acid must, however, be purified before it can be polymerized to form polymeric materials.
The purification of 2,6-naphthalenedicarboxylic acid is considerably more difficult than the purification of a dialkyl-2,6-naphthalenedicarboxylate primarily due to the low solubility of 2,6-naphthalenedicarboxylic acid in most ordinary solvents, and to its high melting point. In the aforementioned U.S. Pat. No. 4,933,491, for example, 2,6-naphthalenedicarboxylic acid was purified only after reacting the 2,6-naphthalenedicarboxylic acid with a lower alkanoic anhydride to produce a component that is soluble in excess alkanoic anhydride. The "solubilized" 2,6-naphthalenedicarboxylic acid was optionally treated with one or more purification procedures. Xu et al. (Chemistry of Synthetic High Polymers, Vol. 10, pp. 107-11, 1984, Chemical Abstracts CA 102: 185547z) describes the purification of 2,6-naphthalenedicarboxylic acid by routine sublimation, recrystallization or distillation as inefficient and difficult due to the poor solubility of 2,6-naphthalenedicarboxylic acid and also because the impurities present, having similar properties, adhere to each other. U.S. Pat. No. 3,649,680 to McNarney discloses a process for purifying aromatic carboxylic acids wherein a mixture of water and an alkanol are added to an impure carboxylic acid paste, the carboxylic acid is separated from the alkanol/water mixture, and the purified carboxylic acid is subsequently washed with water. U.S. Pat. No. 3,671,578 to Ogata discloses a process for preparing 2,6-naphthalenedicarboxylic acid wherein the monoalkali salt of 2,6-naphthalenedicarboxylic acid is heated in water or a water-containing organic solvent, causing disproportion thereof into 2,6-naphthalenedicarboxylic acid and the dialkali salt of 2,6-naphthalenedicarboxylic acid. U.S. Pat. No. 3,888,921 to Yamamoto et al., discloses a process for purifying 2,6-naphthalenedicarboxylic acid wherein an aqueous solution of a dialkali salt of crude 2,6-naphthalenedicarboxylic acid is prepared, then 40 to 97 mole percent of the dialkali salt is precipitated as a monoalkali salt while maintaining the pH of the aqueous solution at a value of not lower than 6.3, and converting the precipitate to 2,6-naphthalenedicarboxylic acid. It is disclosed in the Yamamoto et al. patent that the aqueous solution of the dialkali salt of 2,6-naphthalenedicarboxylic acid can be at a temperature of 60.degree. C.-350.degree. C. in the presence of potassium or sodium hydroxide, and it is disclosed that the solution can be treated with a reducing agent such as hydrogen gas, sodium dithionite, lithium aluminum hydride or sodium borohydride. U.S. Pat. No. 3,781,346 to Norton discloses a process for purifying naphthalene carboxylic acids comprising reacting a solid ammonium salt of the acid with steam at a temperature of from about 200.degree. C. to about 300.degree. C. U.S. Pat. No. 4,794,195 to Hayashi et al. discloses that as it is impossible to purify crude naphthalenedicarboxylic acid to a high purity only by crystallization, and that it is necessary to combine the method of crystallization with other methods such as thermal treatment, oxidative treatment or reductive treatment. However, no specific means for conducting such treatments on 2,6-naphthalenedicarboxylic or other naphthalenedicarboxylic acid is disclosed. U.S. Pat. No. 3,584,039 to Meyer discloses a process for preparing fiber-grade terephthalic acid by catalytic hydrogen treatment of dissolved impure terephthalic acid.
Methods for purifying a naphthalenedicarboxylic acid, such as those described above, that require an alkanoic anhydride or an alkali metal or ammonium salt to solubilize the naphthalenedicarboxylic acid have their drawbacks in that the anhydride or salt that is formed has to be converted to the free acid. Thus, the art needs an improved method for purifying a naphthalenedicarboxylic acid and the present invention provides such a method.